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Knob and tube electrical of wiring has been installed in homes from the 1920s right up into the 1970's in some jurisdictions. Only a hot and neutral wire are provided. The circuit has no electrical ground path. The individual electrical wires are wrapped in a rubberized cloth. That
was fine when the wires remained suspended in air. Wherever the knob and tube circuit wires pass through building framing
lumber a ceramic tube is used to insulate the wire from the wood. Where the knob and tube wiring is surface mounted in a building it is attached using a ceramic "knob". These ceramic knobs are a good insulator provided they have not been damaged or modified. Knob and tube electrical circuits are not "illegal" and there is not a code requirement that they be replaced. However this wiring method is considered obsolete.

Some issues:

No electrical ground is provided
- the circuit is less safe than a modern grounded electrical circuit
and appliances and devices that use a grounded plug should not be
connected on an un-grounded circuit.

The knob and tube wiring may have become damaged
by age, exposure to leaks, or to chewing rodents. In attics, for
example, we often see that this wiring has been damaged by having been
stepped-on or by chewing rodents.

The safety of the knob and tube circuit may have been affected by building changes such as adding insulation and modifications to the original circuit.